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Date:   Wed, 29 Sep 2021 19:48:31 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Logan Gunthorpe <logang@deltatee.com>
Cc:     linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org,
        linux-block@vger.kernel.org, linux-pci@vger.kernel.org,
        linux-mm@kvack.org, iommu@lists.linux-foundation.org,
        Stephen Bates <sbates@raithlin.com>,
        Christoph Hellwig <hch@lst.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Christian =?utf-8?B?S8O2bmln?= <christian.koenig@amd.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Don Dutile <ddutile@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        Jakowski Andrzej <andrzej.jakowski@intel.com>,
        Minturn Dave B <dave.b.minturn@intel.com>,
        Jason Ekstrand <jason@jlekstrand.net>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Xiong Jianxin <jianxin.xiong@intel.com>,
        Bjorn Helgaas <helgaas@kernel.org>,
        Ira Weiny <ira.weiny@intel.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Martin Oliveira <martin.oliveira@eideticom.com>,
        Chaitanya Kulkarni <ckulkarnilinux@gmail.com>
Subject: Re: [PATCH v3 14/20] mm: introduce FOLL_PCI_P2PDMA to gate getting
 PCI P2PDMA pages
Message-ID: <20210929224831.GA3544071@ziepe.ca>
References: <20210916234100.122368-1-logang@deltatee.com>
 <20210916234100.122368-15-logang@deltatee.com>
 <20210928194707.GU3544071@ziepe.ca>
 <9c40347c-f9a8-af86-71a5-2156359e15ce@deltatee.com>
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On Wed, Sep 29, 2021 at 03:34:22PM -0600, Logan Gunthorpe wrote:
> 
> 
> 
> On 2021-09-28 1:47 p.m., Jason Gunthorpe wrote:
> > On Thu, Sep 16, 2021 at 05:40:54PM -0600, Logan Gunthorpe wrote:
> >> Callers that expect PCI P2PDMA pages can now set FOLL_PCI_P2PDMA to
> >> allow obtaining P2PDMA pages. If a caller does not set this flag
> >> and tries to map P2PDMA pages it will fail.
> >>
> >> This is implemented by adding a flag and a check to get_dev_pagemap().
> > 
> > I would like to see the get_dev_pagemap() deleted from GUP in the
> > first place.
> > 
> > Why isn't this just a simple check of the page->pgmap type after
> > acquiring a valid page reference? See my prior note
> 
> It could be, but that will mean dereferencing the pgmap for every page
> to determine the type of page and then comparing with FOLL_PCI_P2PDMA.

It would be done under the pte devmap test and this is less expensive
than the xarray search.

Jason